There is a variety of situations where it is desirable to generate ice pieces having uniform predetermined physical characteristics. For example, the size and shape of a piece of ice will determine its melt rate and handling properties.
A less familiar but economically important concern arises in connection with the ice cycle water distillation process, where that property of the ice crystal lattice that enables it to exclude foreign matter is exploited to distill fresh water from brine or otherwise polluted water. In any such case, it is essential to grow crystals that are free of brine-entraining flaws and are large enough to reduce the problem of clinging brine to manageable proportions.
The common method for controlling the physical shape and size of ice pieces is to grow them in molds for example, or on solid surfaces from which ice sheets are removed and broken up. These methods are satisfactory for small volume applications where cost and complexity are not major concerns, and where pure feed water is available or where purity is not an important consideration.
In the case of an ice crystal water distillation apparatus, these solid surface cooling processes are typically too expensive. In addition, the uneven heterogeneous crystal seeding and ice growth process is not well suited to the task of creating flaw free crystals.
An alternative approach for generating ice involves the "non-contact" processes. One example is the flash freezing process, where water at its freezing temperature is made to boil at a suitably low vapor pressure. This results in the formation of free floating ice crystals that are chemically pure. However, these crystals are varied in size and typically very small. The problem is that the homogeneous seeding and crystallization process is difficult to initiate and control.
For reasons similar to those noted above in connection with ice, there are many other chemicals from which it is desirable to grow large crystals that are free of flaws.